1. Field of the Invention
The present invention relates to a variable gain amplifier for performing waveform equalization of a signal attenuated by a transmission path, at a relay terminal and/or a reception terminal of a communication network.
2. Description of the Related Art
Recently, a large number of LANs (Local Area Networks) have been constructed to cope with increased communication demands. In a LAN, a coaxial cable or an optical fiber is generally used as a transmission path. Of the two types of transmission paths, due to a skin effect, the attenuation in the coaxial cable is increased as the frequency is increased. Since the attenuation amount is increased in proportion to a 1/2 square of the frequency, this is called a .sqroot.f characteristic.
FIG. 10 shows the .sqroot.f characteristic of a 3C-2T (or 3C-2V) type coaxial cable. The attenuation amount L (dB) is represented as follows, assuming that a frequency is f (MHz) and a cable length is l (km): EQU L.apprxeq.12.7.sqroot.fl [dB] (1)
When the coaxial cable is used as a transmission path, a signal passing through the path is subjected to attenuation of an amplitude as well as a waveform distortion. Therefore, at a receiving side, a variable gain amplifier is provided, which has an amplification characteristic for compensating for the .sqroot.f characteristic and which performs feedback control in accordance with a change in cable length so that the signal level installed is maintained as a constant, and performs waveform equalization of a reception signal.
FIG. 11 shows an arrangement of a conventional variable gain amplifier of this type. This amplifier includes varactor diode CD whose capacitance is externally controlled by variable control voltage Vcd, thereby compensating for the amplitude-frequency characteristic of reception signal input Vi.
In this case, an emitter circuit of transistor TR is equivalently represented as shown in FIG. 12, and its impedance Z(f) is represented by the following equation, assuming that an emitter resistance is RE=aR, and that the equivalent capacitance of the series-connected C* and Cd is C, where Cd denotes the capacitance of diode CD: ##EQU1## AC output vo of the above variable gain amplifier is represented as follows in accordance with equation (2) ##EQU2##
FIG. 13 shows an amplitude frequency response characteristic of amplification degree vo/vi obtained from equation (3). As is apparent from FIG. 13, the amount ((1+a)RL/aR) of increase in gain, the pole (1/CR), and the zero point (1/CR(1+a)) in a high-frequency range can all be set to be prescribed values by suitably selecting a and R. In practice, a proper number of variable gain amplifiers are cascade connected, so that the accuracy of the .sqroot.f increasing characteristic in the total amplitude frequency response characteristic is improved.
Assuming that the cable length is changed from l.sub.1 to l.sub.2, attenuation amount L is given as follows in accordance with equation (1): EQU L=12.7l.sub.2 .sqroot.f=12.7l.sub.1 .sqroot.(l.sub.2 /l.sub.1).sup.2 f (4)
As is apparent from equation (4), changing the cable length by .DELTA.l (=l2-l1) is equivalent to changing the frequency from f to f(l.sub.2 /l.sub.1).sup.2. Therefore, by changing C in FIG. 13, the frequency response curve can be slid from .circle.1 to .circle.2 , so that the compensation characteristic can follow the change in the actual cable length. In this case, the relationship between the changes in c and l is represented by the following equation: EQU c.sub.2 /c.sub.1 =(l.sub.2 /l.sub.1).sup.2 ( 5)
However, since the capacitance of the varactor diode changes up to a maximum of ten times, the change in cable length which can be compensated for by the varactor diode is limited to about three times .sqroot.10 times). For this reason, assuming that the maximum cable length is lmax, the range of 1/3lmax to lmax can be compensated, and the range below 1/3lmax cannot be compensated. Therefore, in order to perform automatic equalization throughout the entire cable length, a pseudo line (e.g., an LCR network) of 1/3lmax or more must be provided before the variable gain amplifier and has to be automatically inserted/removed in accordance with the cable length. In this case, however, the size of a circuit is enlarged and the frequency-response compensation control is complicated. Consequently, it is difficult to realize the whole circuitry in an IC arrangement, resulting in a serious disadvantage.
As described above, according to the circuit of FIG. 11, characteristic compensation is performed using a varactor diode. Since a variable range of a capacitance of the varactor diode is not enough, the range of a cable length, within which the .sqroot.f characteristic can be compensated for, is limited. Therefore, in order to achieve an automatically compensation for a short cable, a pseudo line must be provided to be automatically inserted/removed. As a result, the size of a circuit is increased, control is complicated, and an IC arrangement of the whole circuitry is difficult to realize.